A Systematic Presentation
of Organic Phosphorus and Sulfur Compounds James B. Hendrickson
Brandeis University, Waltham, MA 02254 The names. interrelations and oxidation levels of organic compounds of phosphorur or of sulfur frequently tend to cause confusion for students. Avery simple systematic organization of these compounds can serve to clarify the commonly chaotic perception of these molecules. The system consists of grouping them state and extent of carhon substitution. ~ - -bv ." -oxidation ~The procedure is analogous to that previously offered for carbon compounds themselves.' In this system one necsssarily sees all the possible combinations and their relation to each other, clarifying both the chemical and nomenclatural relationships as well as their oxidation state comparisons. ~
though some are known not to be appreciably ionized. Thus, in each type of compound we can in a formal way think of phosphorus as always tetravalent and positively charged.
~~~~
Phosphorus Compounds
Central to creatine. relatine..phosphorus to the - a system . common organic elements (carbon, nitrogen, etc.) is the necessitv to think of phmphorus as tetravalent. like tetravdent nitrogen, tetravalent phosphorus bears a formal positive charge. With simple phosphonium salts, like ammonium saltri, this is obviously so: R4P+X-. In trivalent compounds the phosphorus atom carries an unshared pair of electrons. This may he understood as the conjugate base of the corresponding, trivalent protonated acid: RsPH+ = Rap: H+. The third common form of phosphorus is the phosphoryl group. R3P=0, for which the chief other resonance form is R3P+-0- with again a positively charged tetravalent phosphorus. These three common forms are symbolized and summarized in forms 1-111. Pentacoordinate phosphorus may usually be understood formally as PC15 = PCla+Cl- even ~
+
+5
t
The oxidation state of any tetravalent atom in a covalent molecule is simply the sum of its bonds (n or a) to more electronegatiue atoms, minus the sum of its bonds to less electronegatiue atoms,plus its charge.2 In other words we add the bonds from phosphorus to C and H and give the sum a negative value, while the bonds to N, 0 , and halogens (Hal) are added as positive. This allows a quick computation of the oxidation state ( x ) for phosphorus when they are all added up. The resonance form counted will always
' Hendrickson, J. B., J. CmM. Eouc., 55, 216 (1978). Ferguson L. N., J. CHEM. EoUC., 23, 550 (1946).
0 s
II
P& (PZd Phosphm~cAc.; Phosphates Phosphoryl halides Phosohoramides
Notes: r = number of carbon (alkyl, aryl, seyll attachments
Z = Hal, OH, OR. NH?, NRs
RI'HZx (RPZ21 Alkyl(aryl1-phmphonous acids, halides, amides
"
RzPZx (RzPZI Dialkyliaryll-phorphinic acids, halides. amldes Dialkvliarvll-ohosohi~ialkvlo-ohosohinatesates
\
m
RIP Tetraalkyl(aryl1phmphnnium
Figure 1.
I
Interrelation of all phwphorus ~mpourds. Volume 62 Number 3 March 1985
245
have four bonds and a +1 charge on P. The idea is expressed in a somewhat different. hut eouivalent. wav in the table. Here the bonds from phosphorus a; assumed 1; he either to more electroneeative atoms (7.= N. 0. Hal) or to less electronrnative atoms ( c ~ H )and so the oxidation state (x) is the halance of these, plus a charge of +l. This determination of phosphorus oxidation state makes possible a chart of oxidation state (x) versus number of alkyl (carbon) attachments (r), which is shown in Figure 2. Here each formal tetravalent (and positive) phosphorus is equivalent to forms with an unshared electron pair in place of hydrogen (3) and with -Z implying honds to -Hal, -NR2, -NHR, -NH2, --OR, -OH, or 4-, the latter representing the common phosphoryl group (2). These equivalent, and usually more familiar, forms are shown in parentheses in Figure 1and the family is named below each one; the letter R is used in its ordinary sense of "alkyl," or indeed any attachment of carbon. I t is important to clarify that the number of Z is not the number of attached heteroatoms hut is the number of bonds from phosphorus to heteroatoms, and the same is true of Rdesignations. Hence, in the case of true n-bonds, which are uncommon in phosphorus chemistry, a double hond to one atom is counted as Zz (or R2) as in RNH-P=NR, designated or the very unstable cyanide analogue, HC=P: as PHZ~, designated a t R&H. This treatment of multiple honds is analoeous to the rules in the designation of Rand S for ahsolute stereochemistry in which a-double hond to carbon is counted,as Cz. The value lies in correlating compounds like RNH-P=NR with the parent :PC13 from which it may he made by simple substitution with RNH2; both are classified as H P z ~ with , the same general substitution pattern and oxidation state. The chart in Figure 1 now necessarily shows all possible forms, or families, of phosphorus compounds and their interrelations (extept the uncommon compounds with P-P honds). Within any family the generality -Z is taken to mean any hond to halogen, oxygen, or nitrogen. Thus, like the carboxyl family for carbon, most of the entries in Figure 1have a number of functional variants. The alkyl-phosphonic acid family (RPOZ2) is annotated as RPZ3 (r = 1;x = 3) and includes: the phosphonic acid itself, RPO(0H)z; its acid chloride, RPOC12; its esters, RPO(0R')z; and its amides RPO(NR'2)e. With carhoxylic acids (RCOOH) the analogy is acid chloride (RCOCl), ester (RCOOR') and amide
*
(RCONR'Z), all one interconvertible family a t the same oxidation state (1: = 3). Examples from the chart are shown helow for the families of nhosohinous and ~hosohonous acid derivatives, made by . . re&ci& Z with more electronegative heteroatoms and/or H bv an elrrrron oair. Implicit in thrse families are phosphorus tiutomers, i.e.; compounds related simply by atransfer of hvdroaen to another heteroatom. These . - from nhosohorus . tautomers are readily interconvertible, have the same oxidation state, and in this system employ the same designation. The two tautomers of the acids are included in both examples below. The common phosphoryl group (3P=O,>fi - 0 ) is of course paralleled by the sulfide (>P=S i P S) and selenide as structural analogies.
-
R-P:
RPCI,
/OH
-
-
O 'H
R~H-
R-P-NR,
R-~=NR'
I NR,
1
r=Q l
Nitryl chloride
-1
1
-3
0
--c
AlkyNarylI hydrorylamines
Ammonium Figure 2. Interrelation of all nitrogen compounds.
246
Journal of Chemical Education
h
,
e e e R:INZ(R~N-O) l Amine oxides
in Figure 1, where their locations on the chart show their remlar correlation with oxidation state ( x ) and alkyl or aryl su'bsritution ( r ) . This chart now indeed encompasses in compact form virtually the entire span of the structures, nomenclature and reactions of organic phosphorus compounds; an excellent review of this chemistry is referenced.:'
The interconversions among the entries in Figure 1can take place in three ways. In Figure 1the vertical interconversions are simple oxidation-reduction reactions, interchanging Z and H (or unshared nair) without affectina the level ( r )of carbon &hment. On the chart these reactions are th-m that simply proceed straight up or down. Oxidation of @gPto @3PC12increases the oxidation state from -3 to -1, and reduction of alkylphosphonic acids or halides (RPOZ2) to alkylphosphonous acids (RPZ2)lowers the phosphorus oxidation state from +3 to +l. Reactions which go to the right (Ar = +1) are reactions forming P-C bonds, and these are of two kinds on the chart, simply straight to the right or down to the right. Those which proceed at the same oxidation level require R+ (carbon electrophiles) and imply phosphorus electron-pair displacement of leaving groups from carbon (cf., P-alkylations), as in eqn. (1); in terms of the chart this is a replacement of H by R, i.e., straight to the right. Those which lower the oxidation state at phosphorus (down to the right on the chart) are commonly displacements of halogen from phosphorus hy carbanions, R(e.g., organometallics).In these we see replacement of Z by R as in eqn. (2).
+
-
~ S P : C H ~ +&cH~I(R4P+) r = -3 (R3PH+) r = -3
P(-OW-ire) Phosphorous Acid (phosphiteesters)
Phosphoric Acid P(OR~ (phosphate esters) R-P(O& Alkyl-phosphonic Acid R-P(oR'~ (-phosphonate esters) R@(OH) Dialkyl-phosphinic Acid R#~R') (-phosphtnate esters) [-ic chiorides and amldes]
Alkyl-phosphomus Acid I-phosphonite esters) Dialylslhosphlnous Acid (-phosphinite esters) [+US chlorides and amides]
PO(0H)s WlORh R-PO(0Hh R-POIOR')2 R2PO(OH) R2PqOR')
Nitrogen Compounds The periodic table implies a parallel chart for nitrogen compounds can he made, and this is shown as Figure 2. As in the phosphorus chart the nitrogen is always tetravalent and positively charged, any hydrogen can be replaced by a neutral electron pair, and Z represents bonds to either oxygen or halogen. Although they are more common with nitrogen than
(1)
The nomenclature of organophosphorus compounds is somewhat complicated by the variety of combinations its valence affords. However, a simple table shows how the name endings are formulated. They are also to be found in the chart
,
P(-ic; -ate)
q0Hh
Kirby, A. J.. and Warren. S. G., phorus." Elsevier. 1967.
"The Organic Chemistry of Phos-
2+ SZ, (SOa; S0222) Sulfur trioxide; sulfuric acid Sulfates; sulfuryl chloride
\
2+ HSZB!SO~; s o z 2 ) Sulfur dloxlde; sulfurous acid Sulfites; thionyl chloride
\
\
2+ H~SZ~(SZZ;SO~~-) Sulfur diehloride; sulfoxylates
Alkyl(ary1)-sulfonic acids s u l f o n y l halides -jlfoua\
--c
2+ 11 RSHZp (R-S-2) , Alkyl(ary1)-sulfmc acids -sulfinyl halides
1 \
2+ HJSZ (HSZ; HSOH)
SH4 (H& Hydrogen sulfide Sulfides
He
2+
RSHzZ (R-$-Z) Alkyl(ary1)-sulfenic acids
,
2t
2+
RSZJ (RSOZZ)
A
-
+
2+ R2SZs (RzSOZ) Sulfones
2+ R2SHZ (R?s=o) Sulfoxrdes
-;dfmyl halides -sulfenamides,
2+
RSHJ (R-SH) Tbiols; mereaptans
-
-
2+ 2t R B Z (RzS-0)
1\ -
2+ RISHP (R& Sulfides
21
1
+
RJSH (RJS:) Sulfonwrn
Figure 3. interrelation of all sulfur compounds.
Volume 62 Number 3 March 1985
247
phosphorus compounds, N-N bonds are not in the chart. However, the oxidation states of the nitrogen are still computed in the same way with the bonds t o nitrogen counted as zero. Thus, the oxidation state of each nitrogen in HzN-NHz is x = 0 - (0 3) + 1 = -2, and the two nitrogens in azoxybenzene (bN=NO@) are -1 and +1, respectively.
+
Sulfur Compounds In order for sulfur to conform to the same convention i t must he tetravalent and hear two positive charges. With this proviso we can build a comparable chart for sulfur compounds, shown in Figure 3. Again Z = N, 0, or Hal, and H can be replaced by an electron pair and one lower charge. The familiar sulfonyl group is shown in eqn. (3) with its tetravalent resonance form, and the sulfoxide or sulfinyl group is the first structure in eqn. (4),shown also as its formal conjugate acid with H replacing the electron pair. In eqn. (5) is shown the double conjugate acid of a sulfide. Sulfinic acids are shown in the chart as RSHZ22+ and translated into more common re~resentationin e m . (6) either hv re~lacineH with an elecin this wa; two tautomeric tron pair or by changing z into 0; forms of the sulfinic acid are generated, and indeed both are implicit in its chemistry, and easily interconvertible. It must be emphasized that sulfur does not actually hear the two positive charges. They represent a convenience for easy oxidation state determination and com~arisonof families. The actual, equivalent neutral compo;nds are simply replacements of H by electron pair or S+-Z by S=O.
0R-S-H
12+ + +
b-
R-S-H
fi 1 I
0
Oxldatlon States of Sullur and Phosphorus Attachments
A+% negative:
Block, E., "Reactions of Organosulfur Compounds." Academic Press, 1978. 248
Journal of Chemical Education
--C
r
-H
h
Oxidation State (4
Phosphorus: x = r - ( r (or nitrogen) Sulfur:
+h)+l x =r
- (r
+h+2 more
For a simpler computation: (if no P-P, N-N, Phosphor~s:x = 2z -3
ar S-S
bonds):
(nitrogen) Sulfur:
x =2r
-2
SO3 (equ. (8)). Carbon electrophiles form C-S bonds by reaction with electron pairs on sulfur, i.e., replacement of H with R o n the chart, and so proceed with no change in sulfur oxidation state. In this way alkylation of sulfides (RzSHz2+) produces sulfonium salts (R3SH2+)as in eqn. (9).
Tautumerism and ambident alkyhtion must all maintain the same oxidation state in each of these elements. Thus, hoth N -and O-alkylation u t nitrite b n must have thesame oxidatlon state, as shown in elm. (10). Similarlv diarvl~hosohinic -. . acid tautomerism and aikylation is collected in eqn. (11): phosphorus maintains x = -1 throughout. The Arbuzov rearrangement, a major reaction of phosphorus chemistry, is implicit in eqn. (11) as the conversion of R2P-OR' to R2P(=O)Rf. Similar tautomerism of a sulfur compound is shown in eqn. (6) and ambident alkylation of sulfinates in eqn. (12). R-X
The interconversions of organic sulfur compounds4 follow the same pattern as with phosphorus compounds. Vertical steps on the chart in Figure 3 show simple oxidations and reductions (Z and H interchange), as in the oxidation of sulfoxides to sulfones. Reactions to the right on the chart create C-S bonds. Reaction with carbon oucleophiles constitutes a reduction of sulfur, i.e., replacement of Z with R on the chart as in the conversion of sulfinyl halides (RSHZ22+)to sulfoxides (RzSHZ2+) with organometallic carbauions (eqn. ( I ) ) , or sulfonation of aromatics to arylsulfonic acids (RSZ32') with
(
Number
+
-
:0-R=0 (HNZ3+)r
--. = +3
- H
0-
0
II R-S=O
R-
I
R'
(RzSZz2+) x = +Z
1 q-R'X
t
99-
R-S~O-
other, and their oxidation states. Only two sets of equivalencies are used throughout: P+ 2 P+-Z P:I P t H S=O = S+-Z S: S+-H
R'X
-
4
R-S-R' (RSHZz2+)r = +2
(12)
Summary
With this simple system we see all possible families of phosphorus and sulfur compounds, their relationships to each
The oxidation state is computed as shown in the table. Reactions with carhon nucleophiles constitute reduction of sulfur or phosphorus, while proton tautomerism or reactions with carbon electrophiles (alkylation, acylation, etc.) maintain the same oxidation state. Finally, a completely parallel treatment is, of course, valid for lower elements in the periodic table: arsenic and antimony are treated like phosphorus; and selenium and tellurium, like sulfur.
Volume 62 Number 3 March 1985
249
